1. Technical Field
This invention relates to an apparatus for converting linear displacements to angular displacements. In particular, this invention provides a means for converting a unidirectional displacement of a rack gear assembly to an angular shaft displacement of selectable direction.
2. Background Art
The use of a rack and pinion gear combinatino for the conversion of linear motion to angular motion is well known. In the standard rack and pinion combination a linear displacement of the rack gear corresponds to an angular displacement of the pinion gear in a given direction. If an angular displacement in the opposite direction is desired, the rack gear must be displaced in the opposite direction to the first linear displacement. The controlling mechanism therefore must have two degrees of freedom, one for a forward direction and one for the opposing or reverse direction.
Such controlling mechanisms have been used and are suitable for applications where the operator has one hand or foot available to do nothing but control the mechanism. One such device is disclosed in PIERCE, U.S. Pat. No. 2,957,352, dated Oct. 25, 1960. In PIERCE the gear shift and throttle control device disclosed teaches a bidirectional actuation layer cooperatively engaging a rack and pinion combination. This device is primarily used in marine applicatios for the controlling of marine propulsion engines.
A second marine engine control device is disclosed in MANZOLILLO, U.S. Pat. No. 2,759,578. The MANZOLILLO patent teaches a bidirectional lever cooperatively engaged with several pinion gears for control of the throttle and direction of the marine propulsion engine.
However, an interesting problem has arisen in the applicaton of a rack and pinion combination to a hydrostatic throttle control. As background information necessary to put the full ramifications and advantages of this invention into perspective. Reference is made to FIG. 1 which discloses a typical hydrostatic drive system currently in use today in numerous applications including, but not limited to, forklift trucks, motorized carts, and other industrial specialty vehicles such as powered cement buggies. Additional devices which incorporate hydrostatic control systems include, walk behind devices ranging from lawn mowing equipment to sidewalk snow plows. Referring to FIG. 1, engine 30 drives hydraulic pump 31 by means of a mechanical connection through engine shaft 39. Engine 30 is, in most applications, operated at a constant rpm, and is controlled by a locking throttle or revolutions limiter of some sort. Pump 31 typically is of a variable swashplate design wherein the angle of the swashplate, relative to the pump shaft, determines the volume and direction of hydraulic fluid flow.
Control valve 34 is used to control the angle of the swashplate to direct high pressure hydraulic fluid into either first hydraulic motor line 32 or second hydraulic motor line 33. Which hydraulic line is used for the high pressure supply, will determine the direction of rotation of hydraulic motor 38. Additionally control valve 34 is used to regulate the amount of high pressure hydraulic fluid that is supplied to hydraulic motor 38, by controlling the amount of angular deflection of the swashplate, thus not only regulating the direction of rotation, but also the speed of rotation of hydraulic motor 38. Control lever 35 is typically provided for hydraulic control valve 34 to provide for manual and/or semiautomatic control of valve 34.
In hydrostatic drive applications currently in use for forklift trucks, control lever 35 is a foot operated rocker switch for use by the operator. If he rocks the switch forward with his foot, the forklift will move in a forward motion, if he rocks his foot back, the forklift will move in reverse.
The problem with hydrostatic drive systems is that the change from a forward direction to a reverse direction is as close to instantaneous as can be possible, considering concepts of momentum and only minimal time lags in the hydraulic system. This results in tremendous decelerations and reversing of direction in the event of an operator of a forklift truck moving in one direction were to accidentally move the control lever from one direction to the other. This obviously creates dangers of loss of a load being carried, and/or injury to personnel or property.
Dividing the control valve functions of directional and throttle control between two separate control valves, one controlling the direction of hydraulic fluid flow to the motor and the second, being a throttle valve for controlling the amount or volume of fluid flow is not a satisfactory solution. First, it is a complicated system which requires the operator to dedicate at least two limbs, usually one hand and one foot to direction and speed control at the same time when the operator may likely be required to operate other controls such as the forklift controls on a forklift truck. Secondly, such systems, by their design, permit the operator to first open the throttle valve, before the directional control valve is positioned for either forward or reverse motion. Unless very sophisticated and expensive linkage interlocks are installed, it is still possible, using two valves, to inadvertently and almost instantaneously reverse the direction of the hydrostatic drive system. What is needed is a device which insures, by its inherent design, that the hydrostatic drive system must be in the neutral position before the operator can reverse the rotational direction of the hydrostatic motor.
In the case of walk behind vehicles such as cement buggies, snow blowers and the like, the present systems require a bidirectional action for a control lever mounted upon a handle bar since the use of a foot controlled rocker switch is not feasible for an operator walking behind the vehicle. As a result, these controls are usually located on handlebars. Cable and lever type linkage for bidirectional control systems on handlebars are awkward to use especially if the operator is required to maintain a forward pressure on a lever while walking behind the vehicle. In practice it has been found that it is difficult to maintain relative stability between the vehicle and operator, when the operator has to maintain constant pressure on a control lever when walking behind the vehicle. This is especially true when traversing over uneven or inclined surfaces. Generally speaking, it is difficult for an operator to gain necessary skills to maneuver a hydrostatically driven walk behind vehicle with sufficient skill to insure safety and proper operation. What is needed is a caliper type handlebar throttle control which can be used for both forward and reversing operations.
And again, as in the case of riding vehicles such as forklift trucks, there must be some mechanical system to prevent the operator from shifting fromthe forward to reverse direction without first insuring that the hydrostatic drive system is in the neutral position.
Other problems arise when applying the bidirectional throttle control lever to hydrostatic drive applications. Hydrostatically driven machinery typically require the operator to control several different functions related to the utility purpose of the vehicle at any given time. It is therefore desirable to have a throttle actuation means which requires very little attention and effort expended on the operator's part. Because a bidirectional lever requires the operator to push it one direction to proceed in that direction and to pull it in the opposite direction to proceed in the opposite direction, it requires constant attention on the part of the operator. This is especially true if the hydrostatic drive control system is radically different from the normal mechanical control systems, such as those for automobiles, trucks or motorcycles, which an unskilled or semiskilled laborer may be accustomed to operating. If the hydrostatic drive system controls resemble and are similar in function to the usual mechanical control systems, then it is easier and quicker for a laborer to learn how to skillfully operate the system, and productivity will be increased.
What is needed is a device that requires minimum attention on the operator's part, resembles typical mechanical control systems, and has built into it a safety device prohibiting the sudden directional changes associated with bidirectional lever control systems for hydrostatic drives.
Accordingly, it is an object of this invention to provide an apparatus for the conversion of a unidirectional displacement to a bidirectional displacement of selectable direction. The advantage of this is that it facilitates the use of a caliper throttle actuating lever which only requires a unidirectional displacement to control both forward and reverse operations.
A second object of this invention is to provide a safety mechanism which is inherent in the standard operation and selection of the directoin of motion which requires the hydrostatic control valve to be at a neutral or closed, non-driving, position when a change is made from forward to reverse.